1. Field of the Invention
The present invention relates to a radiation-curable acrylated semi-crystalline hyperbranched polyurethane oligomer and a preparation method thereof.
2. Description of the Prior Art
Powder coatings have been developed rapidly in recent years because of several advantages such as being solvent-free, having no volatile organic compound emissions, and having recyclable fallen wastes, a high availability of raw materials and ease of operation. Traditional thermosetting powder coatings typically, however, need to be cured for 15 to 30 minutes at 160 to 180° C., which causes the thermal deformation of most plastic substrates, while discoloration and scars may also occur at wood substrates due to such a high curing temperature. The thermosetting powder coatings will be required to have much higher reactivity if the curing temperature of the coatings is to be decreased, thereby resulting in the problem of chemical instability in storage and transport. Therefore, conventional thermal-set powder coatings are only suitable for metal substrates. Compared to the thermosetting powder coatings, the melting and curing of radiation-curable powder coatings are two separate processes, i.e., powder particles are firstly melted and levelled at about 100° C. with infrared radiation, then irradiation cured, and thus a flat and smooth film surface is obtained. Moreover, as radiation-curable powder coatings are cured rapidly at low temperature so that energy is efficiently saved, they are applicable in a wide range of substrates, especially thermal-sensitive substrates. Thus, it has been the tendency in the field of coatings to study and develop an oligomer useful in radiation-curable powder coatings.
U.S. Pat. Nos. 5,789,039 and 5,935,661 disclose the use of a radiation-curable, especially ultraviolet-curable, solid oligomer system in powder coatings, which is mainly composed of cationic-cured resins, especially epoxy resins, including bisphenol A and aliphatic-type (including the types of hydrogenated bisphenol A, methacrylic diglycidyl ether and acrylic diglycidyl ether) epoxy resins. Other auxiliaries need to be added in such oligomers to achieve levelling at about 100° C., so that the oligomers can be applied onto thermal-sensitive substrates.
U.S. Pub. Pat. Nos. 2002/0099127 A1 and 2003/0130372 A1 disclose the use of a radiation-curable, especially ultraviolet-curable, solid oligomer in powder coatings, which mainly comprises two components: 60 to 90 wt % of amorphous polyurethane acrylate, and 10 to 40 wt % of crystalline polyurethane acrylate, the mixture of both having a glass transition temperature (Tg) of at least 35° C. The amorphous polyurethane acrylate is prepared by the reaction of hydroxy-terminated polyester (Tg=35 to 80° C.) and polyisocyanate, followed by the end-capping of the acrylates. The crystalline polyurethane acrylate is prepared by the reaction of crystalline hydroxy-terminated polyester and polyisocyanate, followed by the end-capping of the acrylates. The crystalline hydroxy-terminated polyester is prepared by the reaction of a long chain diacid and a long chain diol. The radiation-curable oligomers disclosed in these two patents are linear amorphous polymers with a high glass transition temperature, and have a high melt viscosity, where viscosity decreases very slowly at a temperature higher than Tg so that it is difficult to level. Moreover, the oligomers have a low radiation-curing rate due to their lower functionality.
Chinese Patent ZL 00119066.0 discloses a radiation-curable, especially ultraviolet-curable, acrylated semi-crystalline dendritic polyether amide oligomer, but the synthesis of the dendritic polymer is complicated and expensive, thereby limiting its application.
Macro molecules (Vol. 36, p. 613-620, 2003) discloses a synthesis method of hyperbranched polyurethane, but the hyperbranched polyurethane obtained by this method is an amorphous polymer, with no methacrylate or acrylate functionality and no long carbon chain or benzene ring or naphthalene ring at the terminals, and thus, it cannot be cured by a radiation method.